This invention relates generally to light-emitting diodes and their fabrication. More particularly, it concerns light-emitting diodes fabricated from semiconducting (conjugated) polymers which are soluble in common organic solvents, and yet more particularly to the fabrication of such diodes on flexible polymer substrates.
Solid-state light-emitting diodes (LEDs) have found widespread application in displays, as well as in a variety of less common applications. Currently, LEDs are fabricated from conventional semiconductors; for example, gallium arsenide (GaAs), typically doped with aluminum, indium, or phosphorus. Using this technology, it is very difficult to make large area displays. In addition, the LEDs made of these materials are typically limited to the emission of light at the long wavelength end of the visible spectrum. For these reasons, there has been considerable interest for many years in the development of suitable organic materials for use as the active (light-emitting) components of LEDs. (See references 1-6). The need for relatively high voltages (i.e., voltages incompatible with digital electronics) for the onset of light emission has been a hindrance to the commercialization of LEDs fabricated from organic materials.
The utilization of semiconducting organic polymers (i.e., conjugated polymers) in the fabrication of LEDs expands the use of organic materials in electroluminescent devices and expands the possible applications for conducting polymers into the area of active light sources, (see Reference 7) with the possibility of significant advantages over existing LED technology. Controlling the energy gap of the polymer, either through the judicious choice of the conjugated backbone structure or through side-chain functionalization, should make possible the emission of a variety of colors throughout the visible spectrum.
In the prior art, Tomozawa et al (see Reference 8) disclosed diodes fabricated by casting semiconducting polymers from solution.
Also in the art, Burroughs et al (see Reference 7) disclosed a multi-step process in the fabrication of LED structures characterized as follows:
1) A glass substrate is utilized. The substrate is pre-coated with a transparent conducting layer of indium/tin oxide (ITO). This ITO coating, having high work function serves as the ohmic hole-injecting .electrode.
2) xe2x80x2 A soluble precursor polymer to the conjugated polymer, poly(phenylene vinylene), PPV, is cast from solution onto the substrate as a thin, semitransparent layer (approximately 100-200 nm).
3) The precursor polymer is converted to the final conjugated PPV by heat treating the precursor polymer (already formed as a thin film on the substrate) to temperatures in excess of 200xc2x0 C. while pumping in vacuum.
4) The negative, electron-injecting contact is fabricated from a low work function metal such as aluminum, or magnesium-silver alloy; said negative electrode acting as the rectifying contact in the diode structure.
The resulting devices showed asymmetric current versus voltage curves indicative of the formation of a diode, and the diodes were observed to emit visible light under conditions of forward bias at bias voltages in excess of about 14 V with quantum efficiencies up to 0.05%.
The methods of Burroughs et al, therefore, suffer a number of specific disadvantages. Because of the use of a rigid glass substrate, the resulting LED structures are rigid and inflexible. The need for heating to temperatures in excess of 200xc2x0 C. to convert the precursor polymer to the final conjugated polymer precludes the use of flexible transparent polymer substrates, such as, for example, polyethyleneterephthalate, polystyrene, polycarbonate and the like, for the fabrication of flexible LED structures with novel shapes and forms. The need for heating to temperatures in excess of 200xc2x0 C. to convert the precursor polymer to the final conjugated polymer has the added disadvantage of possibly creating defects in the conjugated polymer and in particular at the upper surface of the conjugated polymer which forms the rectifying contact with the low work function metal.
Thus, the ability to fabricate light-emitting diodes from organic materials and in particular from polymers, remains seriously limited.
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6. M. Nohara, M. Hasegawa, C. Hosohawa, H. Tokailin, T. Kusomoto, Chem. Lett. 189 (1990).
7. J. H. Burroughs, D. D. C. Bradley, A. R. Brown, R. N. Marks, K. Mackay, R. H. Friend, P. L. Burns and A. B. Holmes, Nature 347, 539 (1990).
8. H. Tomozawa, D. Braun, S. D. Phillips, R. Worland, A. J. Heeger, and H. Kroemer, Synth. Met. 28, C687 (1989).
9. F. Wudl, P.-M. Allemand, G. Srdanov, Z. Ni, and D. McBranch, in Materials for Non-linear Optics: Chemical Perspectives (to be published in 1991).
10. S. M. Sze, Physics of Semiconductor Devices (John Wiley and Sons, New York, 1981).
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It is accordingly an object of the present invention to overcome the aforementioned disadvantages of the prior art and, primarily, to provide light-emitting diodes fabricated from semiconducting polymers which are soluble in the conjugated form and therefore require no subsequent heat treatment at elevated temperatures.
It is additionally an object of the present invention to utilize the processing advantages associated with the fabrication of diode structures from soluble semiconductor polymers cast from solution to enable the fabrication of large active areas.
It is additionally an object of the present invention to provide light-emitting diodes fabricated from semiconducting polymers using flexible organic polymer substrates.
It is additionally an object of the present invention to provide methods for the fabrication of light-emitting diodes fabricated from semiconducting polymers which turn on at bias voltages compatible with digital electronics (i.e., at voltages less than 5 volts).
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art on examination of the following, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.
In one aspect this invention provides a process for fabricating light-emitting diodes (LEDs). In this embodiment the process involves a process for fabrication of light-emitting diodes which emit visible light. This process comprises the steps of:
i) precoating a substrate with a transparent conducting first layer having high work function and serving as an ohmic hole-injecting electrode;
ii) casting upon the first layer directly from solution, a thin transparent layer of a soluble conjugated polymer; and
iii) fabricating a negative, electron-injecting contact onto the conjugated polymer film. This contact is formed from a low work function metal and acts as the rectifying contact in the diode structure.
In another embodiment, an alternative process for fabricating light-emitting diodes which emit visible light is provided. This process comprises the steps of:
i) casting a free-standing, semi-transparent film of a soluble conjugated polymer from solution, said film serving as a luminescent, semiconducting polymer and simultaneously as a substrate;
ii) coating the free-standing, conjugated polymer film on one side with a transparent conducting first layer having a high work function and serving as the ohmic hole-injecting electrode; and
iii) fabricating a negative electron-injecting contact onto the other side of the conjugated polymer film. This contact is made of a low work function metal and acts as the rectifying contact in the diode structure.
In yet an additional embodiment this invention provides a process for making oriented polymer-based LEDs which emit polarized visible light. This process includes the steps of:
i) gel-processing a soluble conjugated polymer as a member of an admixture with ultra-high molecular weight carrier polymer. The gel-processed mixture is formed into an oriented, free-standing film in which the conjugated polymer is chain-aligned. This chain-aligning resulting in polarized luminescence for the polymer.
ii) coating the free-standing, oriented polymer film on one side with a transparent, conducting first layer having high work function and serving as an ohmic hole-injecting electrode, and
iii) fabricating a negative, electron-injecting contact onto the other side of the conjugated polymer film. This contact is fabricated from a low work function metal and acts as the rectifying contact in the diode structure.
In another general aspect this invention provides the LEDs fabricated by any of these processes. In a more particular aspect of this invention, the LED devices employ poly(2-methoxy,5-(2-ethyl-hexyloxy)-1,4-phenylene vinylene), MEH-PPV, as the conjugated polymer. MEH-PPV offers the advantage of being a conjugated polymer which is soluble in organic solvents. LED device fabrication is simplified because of the direct casting of the conjugated polymer from solution.
Surprisingly, it was found that by using calcium as the low work function rectifying contact, and by using ITO coated PET films as the substrate, flexible LED structures are fabricated which benefit from the excellent mechanical properties of both the polymer substrate and the conjugated polymer semiconducting layer and which exhibit the advantageous characteristics of a turn-on voltage reduced to 3-4 volts (i.e TTL compatible), and a quantum efficiency which is improved by more than an order of magnitude to values of approximately 1%.
Specific advantages of this invention over the prior art include the following:
(i) Because the luminescent semiconducting polymer is soluble in its final conjugated form, there is no need for heat treatment at elevated temperatures. This greatly simplifies the fabrication procedure and enables a continuous manufacturing process.
(ii) Since the luminescent semiconducting polymer layer can be cast onto the substrate directly from solution at room temperature, the LED structure can be fabricated on a flexible transparent polymer substrate. These polymer films are manufactured as large area continuous films. Thus, the use of flexible polymer films as substrate enables the fabrication of large area polymer LEDs using either a batch process or a continuous process.
(iii) The use of Calcium as the low work function contact onto MEH-PPV as the luminescent polymer leads to unexpected improvements in the efficiency of the device and in the compatibility of the device with modern digital electronic circuitry.